To analyze the systems working with solar energy, it is required that values of the solar radiation belonging to the region have been known. The global solar radiation on a horizontal surface has been measured by the Turkish State Meteorological Service over all of the country, while the diffuse solar radiation has not been measured. In this study, empirical correlations are developed to establish a relationship between the monthly average daily diffuse fraction (H-d/H) and monthly average daily diffuse coefficient (H-d/H-o) with the monthly average daily clearness index (K-T = H/H-o) for the province of Erzurum, Turkey. The six new models for estimating the monthly average daily diffuse solar radiation on a horizontal surface for Erzurum are validated, and the new models are then compared with the 10 models available in the literature. In order to indicate the performance of the models, the statistical test methods of relative percentage error, mean bias error, mean absolute bias error, mean relative error, root mean square error, and correlation coefficient are used. The results indicated that the correlations developed in the study predict diffuse solar radiation reasonably well for future projections.